The present invention relates to calenders or analogous machines in general, and more particularly to improvements in calender rolls or analogous rotary bodies which consist, at least in part, of magnetizable material. Still more particularly, the invention relates to improvements in calender rolls or analogous rotary bodies which are equipped with means for preventing, controlling or reducing the extent of flexure or bending of their components.
It is well known that the rotary component (e.g., a hollow cylinder) of a calender roll tends to flex in the middle owing to its weight as well as in response to the pressure which is applied by the rotary component(s) of the neighboring roll(s) and/or by the material (e.g., a paper web) which is caused to run through the nip of two neighboring calender rolls in a paper processing line. The extent of flexure of the rotary component can be quite pronounced, especially when the roll is long and very heavy. Such rolls are often employed in calenders for surface treatment of running paper webs as well as in machines or production lines which turn out and/or process webs, strips or tapes consisting of metallic foil, plastic foil or textile material. Flexing of the rolls is evidently undesirable when the thickness of a running web is to remain constant, as considered in the axial direction of the rotary body, regardless of whether the running web is to be smoothed, condensed, provided with a pattern, heated, cooled and/or subjected to two or more simultaneous treatments. In most instances, flexing of the rotary body causes a reduction of the thickness of a running web in one or more regions between its marginal portions.
It is already known to provide a calender roll or an analogous rotary body with means for opposing the flexure of its rotary component, e.g., the flexure of the hollow rotary cylinder which surrounds the supporting shaft of a calender roll. For example, such means may include a device which establishes a high-pressure hydraulic field between a stationary shaft and a rotary cylinder which surrounds the shaft with clearance. It has been found that such proposal is reasonably satisfactory when the RPM of the cylinder is relatively low. However, when the cylinder is driven at an elevated speed, the viscosity of the hydraulic fluid disturbs the aforementioned pressure field to such an extent that the field is incapable of preventing or adequately reducing the bending or flexing of the median portion of the cylinder.
It was further proposed to prevent or reduce the extent of flexure of a hollow rotary cylinder which surrounds a fixed stationary shaft by installing a row of discrete magnets at the underside of the shaft. The row is parallel to the axis of the shaft and the magnets tend to lift the rotary cylinder so that the lowermost portion of the cylinder tends to move radially of and toward the peripheral surface of the shaft. However, when the cylinder rotates, the roll causes the generation of pronounced eddy currents which counteract at least part of the magnetic compensating (flexure reducing or preventing) force and lead to often undesirable as well as uncontrolled heating of the cylinder.
It is further known to construct magnetic bearings for rotary shafts or the like in such a way that each bearing comprises a magnetic supporting member for a series of electromagnets, as considered in the circumferential direction of the shaft. The pole faces of neighboring electromagnets (as considered in the circumferential direction of the shaft) exhibit different polarities. Reference may be had to the German-language publication entitled "SKF Technische Information Nr. 299" (dated Apr. 12, 1977). The just described bearing structure cannot be used in a calender roll or the like to prevent flexure of a rotary component because, when the shaft rotates in the aforementioned magnetic bearings, the alternating pole faces of different polarities cause a reversal of the field which entails considerable losses as a result of development of eddy currents. Such losses (which are attributable to development of eddy currents) can be kept within a certain range by resorting to feedback connectors of iron, sheet metal or the like; however, they cannot be eliminated in full and their remnants are sufficiently pronounced to cause excessive heating of a cylinder which forms part of a calender roll for the treatment of running webs of paper, textile material, metallic foils, plastic foils or laminates consisting of two or more superimposed bands, sheets or strips.